Revert "Raise CMake requirement to 3.0+"

This doesn't work when the partition manager is enabled because it
uses ECM and ECMConfig.cmake fails with an error if the required
CMake is not less than 2.8.13

Added a note here to remember about this.

This reverts commit 2020eb0eb306f7319b2376b06acb8e242e7b7dc3.